Why do I have to apply to the partner institution when I have already been accepted on exchange?

Your initial application for exchange is not the same as the application you will submit to the partner institution.

The partner institution will never see the application you submit to us. This application is for us to internally allocate you a place on exchange. You are then required to submit your official application to the partner - the application requirements vary depending on the country and the institution you are going to. You will be made aware of what you need to do at that stage of the process.

Please bear in mind that whilst unlikely, host institutions do retain the right to decline incoming exchange applications.